Time diversified packet protocol

ABSTRACT

A method of reporting information from a meter interface unit to a receiving device in which a data packet is transferred from the meter interface unit containing meter readings, which are associated with an indicator of the elapsed time since the reading was taken. The receiving device compares this elapsed time value with the actual elapsed time, based upon the internal clock of the receiving device, in order to determine any inaccuracies in the clock of the meter interface unit. In another embodiment, the data packet includes at least two nonsequential meter readings, separated by a multiplicity of reading intervals, on a rolling basis, such that data will not be lost as a result of a temporary obstruction that interferes with the transmission or receipt of meter readings.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application under 35 U.S.C. §120 ofU.S. application Ser. No. 14/268,868, filed May 2, 2014 (currentlypending). U.S. application Ser. No. 14/268,868 is a continuingapplication under 35 U.S.C. §120 of U.S. application Ser. No.13/438,545, filed on Apr. 3, 2012 (now U.S. Pat. No. 8,736,460). Thecontents (but not the prosecution histories) of U.S. application Ser.No. 14/268,868 and U.S. Pat. No. 8,736,460 are explicitly incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The invention relates generally to the field of automated meter readingsystems, such as for water, gas, electricity, or chemicals.

BACKGROUND

Utilities and other entities operate distribution systems for water,gas, and electricity to deliver these resources to various consumersconnected to the distribution system, with a meter at each point theresource is removed from system to measure the consumer's usage. Manymetering systems utilize wireless communications modules operativelyconnected to the meter itself (which may be referred to as endpoints,nodes, or the like, and which hereinafter will be referred togenerically as a meter interface unit, or MIU), such that the MIUreports the meter reading electronically to a communications network.The network may include a mobile communications device that collects thetransmitted readings as a utility worker drives within range of themeters, or stationary receivers or collectors designed to receivemessages from a designated set of meters within a certain range. Variousnetwork topologies and technologies exist in the prior art fortransmitting meter readings from an MIU at the meter either through anintermediary device (whether stationary or mobile, including MIU'sacting as an intermediary device in a mesh network) and on to a centraldata collection or processing unit. In this disclosure, the term“collector” shall refer to any device that receives transmissions ofmeter readings from an MIU in an automated meter reading (AMR) system,in any network architecture or topology. In AMR systems, thecommunications module at the meter might transmit a reading on apredetermined interval (a “bubble up” system), or the module mightrespond to a command to report a meter reading from the central host ora nearby receiver or collector. The details of such networks are knownand understood by those of ordinary skill in the art and will not bediscussed in further detail here. In any case, such communicationsystems offer the ability for rapid transmission of meter readings fromthe meter itself back to the central host computer for compilation andanalysis.

Typically, an MIU transmits a data packet containing one or more meterreadings on pseudorandom intervals. The data packet typically containsthe most recent reading or readings, which roll off the packet based onage as more recent readings become available. For example, assume apacket contains data fields for three readings, and readings are takenevery fifteen minutes, but transmitted every five minutes. A given setof three readings (the most recent reading, the reading taken fifteenminutes before the most recent reading, and the reading taken thirtyminutes before the most recent reading) is therefore transmitted threetimes before a new reading is taken and the oldest reading is droppedfrom the packet to make room for the new reading. In this scenario, eachreading is transmitted three times before being put in a new positionand nine times before being replaced by a newer reading.

Data can be lost, however, if there is some problem preventing asuccessful transmission or receipt of the reading before the readingcompletes its progression through the available data fields in thepacket as newer readings are made. These problems include, for example,weather conditions, environmental conditions, an outage in the receivingnetwork, or other interference. Water meters in particular are oftencontained in pits, below ground level, as are the MIUs. If there isheavy rainfall, the pit may fill with water, which interferes with thetransmission capabilities of the antenna. Similarly, a delivery driveror other person may park a vehicle immediately over or next to the meterlocation, which could block transmission of the meter readings. Whilethese weather, environmental, or network conditions are temporary, ifthey were to last more than an hour in the example above, meter readingswould be lost.

As referenced above, MIUs (especially those in water, gas, or chemicalsystems) often operate on batteries, which are intended to last for manyyears. MIUs also are programmed to take certain actions, such asreadings and transmissions, upon certain time intervals, and it may beimportant to know exactly when a reading was made. MIUs thereforecontain clock or timing circuits. While accurate timing is desirable, ahighly precise clock that does not drift over the intended life of theMIU is relatively expensive and may consume more power than a lessexpensive clock. Given that there are often very large numbers of MIUsin a distribution system (tens of thousands), the incremental cost of ahighly precise clock is multiplied by the large volume of unitsrequired. Apart from considerations of cost and power consumption, theclocks in MIUs may drift or lose accuracy because of the widetemperature ranges to which they are exposed, humidity levels, and agingof components. Preventing such drift could require regular testing andmaintenance of the MIUs or better components, both of which add costs tooperation of the system. On the other hand, there typically are farfewer collectors than MIUs, because hundreds or even thousands of MIUsmay be served by a single collector. Collectors are usually connected toa fixed power source. Because they are substantially fewer in number,collectors are more readily maintained and upgraded than MIUs. It wouldbe advantageous to shift the cost, power, and maintenance requirementsof a highly precise clock from the MIU to the collector.

Thus, there exists a need for an AMR system in which the time that meterreadings were made can be determined accurately without incurring thefull cost of a highly precise clock in the MIU. There also exists a needfor an AMR system in which meter readings will not be lost because oftemporary obstructions that interfere with the transmission or receiptof meter readings.

SUMMARY

Embodiments of the present invention satisfy these needs. One embodimentcomprises a method reporting information from a meter in an automatedmeter reading system, in which a collector or other device receivingdata from a meter interface unit (MIU) in communication with the meterreceives a packet of data from the MIU containing at least one meterreading, in which the reading is associated with an indicator reportingthe time elapsed since the reading was taken, based on a clock of saidMIU. Each reading in the packet is associated with the actual time thereading was taken, based upon a clock of the receiving device and theelapsed time indicator. These steps are repeated with a later packetthat contains at least one of the same readings as the first packet,where the elapsed time indicator of this same reading in the laterpacket is different than the elapsed time indicator of this same readingin the first packet. Then, the actual time elapsed between the samereadings, based upon the clock of said receiving device, is comparedwith the reported time elapsed between the same reading, based upon saidelapsed time indicators in order to determine inaccuracies in the timekeeping functions or clock of the MIU. Inaccuracies in the MIU clockthat exceed a predetermined threshold can be reported to a centralcomputing station or data center. In addition, in an AMR system usingtwo-way communication, a command may be sent to the MIU to reset itsclock to the correct value, or a calibration factor may be sent to theMIU for the MIU to improve the accuracy of its timekeeping functions. Inone embodiment, the clock of the receiving device may set in accordancewith data from the global positioning system.

Another embodiment of the present invention comprises a method ofreporting information from a meter in an automated meter reading system,in which the meter is read upon reading intervals and each reading isassociated with a time stamp corresponding to when the reading was made.A packet comprising a plurality of readings and an indicator of timeelapsed since each said reading was taken is transmitted to a collectoror other device, wherein at least two of the readings in the packet arenonsequential and the elapsed time between them exceeds a multiplicityof reading intervals. In one embodiment, the packets are transmitted ona transmit interval, which may be pseudorandom, and there is at leastone transmit interval in each reading interval. Alternatively, there maybe at least one reading interval in each transmit interval. In oneembodiment, the time stamp corresponds to an actual time of day and theelapsed time indicator is the time stamp. In another embodiment, theelapsed time indicator is the difference between the time oftransmission of the packet and the time stamp. The time stamp can be aserialized time value that does not necessarily correspond to an actualtime of day. Each reading in the packet may be associated with theactual time of day corresponding said indicator by a device receivingsaid transmission. In an embodiment where the reading intervals arefixed, the time stamp may correspond to a particular reading interval.In a preferred embodiment, the packet includes at least three meterreadings, which include the two most recent sequential readings fromwhich the most recent flow rate can be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained, by way of example only, withreference to certain embodiments and the attached figures, in which:

FIG. 1 is a schematic of an exemplary automatic meter reading system;and

FIG. 2 illustrates exemplary data packets containing meter readingstaken and transmitted over a defined time period.

DETAILED DESCRIPTION

Embodiments of the present invention provide methods of reportinginformation gathered by an MIU on selected intervals from a meteringdevice in such a way that the information is not lost even if thetransmission or reception of information from the MIU is compromised formany such intervals. Embodiments of the present invention also comprisemethods of providing temporally accurate information regarding readingseven where the clock in the MIU is subject to inaccuracies and drift.

FIG. 1 is a schematic of an exemplary automatic meter reading (AMR)system. The system shown in FIG. 1 is a three-tiered, hierarchicalnetwork topology and is exemplary only; the methods of the presentinvention can be used with any AMR network topology, architecture, orcommunications scheme. As shown in FIG. 1, MIU's 10 are in operativecommunication with meters 8. The meters 8 illustrated in FIG. 1 arewater meters, although they also could be meters to measure electrical,gas, chemical, or other resource usage. The MIUs 10, which typically arebattery powered, communicate with collectors 14 (which could be any typeof receiving device, including another MIU in a mesh networkarchitecture) that is typically mounted upon utility pole 12. Thecollectors 14 preferably have a line power source. The collectors 14communicate through a network, wired or wirelessly, which may be aproprietary network or the internet, by technology well known in theart, to a server or central data station 20, which aggregates andprocesses the data received in the communications from the collectors14.

In one embodiment, an MIU is programmed to make and store meter readingson a predetermined reading interval. As used herein, the term “reading”may refer to an actual reading of the meter value, or a number derivedfrom the meter value, such as consumption, or a percentage ofconsumption over a particular interval. Thus “reading” should beunderstood to mean a direct reading, a consumption, or any value derivedfrom resource usage measured by the meter. The MIU records a time stampcorresponding to when each reading was taken, according to its internalclock. This time stamp may be an absolute time value or a relative timevalue. An absolute time value may be an actual time of day (e.g.,1:12:03 PM) or simply the value of a serialized counter that incrementsin units derived from the internal clock circuit of the MIU. A relativetime value may be an indicator of the elapsed time since some definedevent has occurred, such as the time the preceding reading was taken.Such relative time values may be expressed in units of real time (hours,minutes, seconds) or as a number of increments of a counter derived fromthe internal clock circuit of the MIU. The MIU also transmits one ormore of these meter readings in a data packet to a collector, in amanner well understood in the art, either on a predetermined interval(which may be a pseudorandom interval) or, in some embodiments where theMIU is capable of two-way communications with the collector, in responseto a command to report information from the collector. Upontransmission, the MIU includes an indicator of the elapsed time sinceeach reading. The elapsed time indicator may take many forms, includingan absolute or relative time value associated with each reading in thepacket, or an absolute or relative time value associated with onereading in the packet from which the time values of the remainingreadings in the packet can be derived, based upon the fixed readingintervals at which the readings were made.

In one embodiment, meter readings are made upon fixed reading intervals,according to the internal clock of the MIU. Each data packet transmittedby the MIU includes three readings, an indicator of the elapsed timesince the earliest (oldest) reading in the packet was taken, and anindicator of the reading interval at which each reading was taken. In atypical prior art system, the packet would contain the three most recentreadings, such that if readings were made once per hour, the packetwould contain the most current reading and the readings for the twopreceding hours. If readings were made every fifteen minutes, the packetwould include the current reading and the readings made over thepreceding thirty-minute time period. In order to prevent readings frombeing lost because of a temporary obstruction that interferes with thetransmission or receipt of meter readings, a packet according to thisembodiment of the present invention preferably includes the most recentreading and at least one prior reading that is nonsequential with themost recent reading and where there are a multiplicity of readingintervals between these two readings.

An exemplary embodiment of a data packet is illustrated in FIG. 2. FIG.2 shows the contents of the data packets containing readings made atreading intervals T₀, T₁, and on through T₃₇. Each data packet containsa header and end of packet indicator, as known in the art. In addition,data packets typically contain error correction information, and mayalso contain packet type information, commands, and “housekeeping”information. Those types of information are not material to the presentapplication and are simply indicated by an ellipsis in each data packet.As shown in FIG. 2, if the reading interval were once per hour and onedesired a twelve-hour gap between readings in the packet, starting attime zero T₀, that is, with the very first reading made by the MIU, thepacket would contain the most recent reading R₀, and the remaining datafields in the packet would contain null values because there is not yeta reading taken twelve hours before. The packet also includes anindicator of elapsed time ET₀ since this reading R₀ was taken. If thetransmission interval were every fifteen minutes, this packet containingthe reading R₀ would be transmitted three times before a new reading R₁is taken. After R₁ is taken at interval T₁, the packet contains the mostrecent reading R₁, the elapsed time ET₁ since R₁ was taken, and theremaining data fields in the packet would contain null values becausethere is not yet a reading taken twelve hours before. This process isrepeated until, in this example, twelve hours has elapsed. After twelvehours has elapsed, the most recent reading R₁₂ is taken at interval T₁₂,and the MIU's memory now contains a reading twelve hours old, namely R₀.The elapsed time indicator ET₀ indicates how much time has elapsed sinceR₀ was taken to the transmission of the packet. The elapsed time sincethe R₁₂ reading was taken is therefore ET₀ less twelve readingintervals. (Of course, this process could be altered by basing theelapsed time indicator on the most recent reading and calculating theelapsed time of the earlier readings based the number of elapsed readingintervals.) Similarly, an hour later at T₁₃, the most recent reading R₁₃is transmitted along with R₁ and ET₁. If the packet contained threereadings, according to this embodiment, where the transmitted readingsare twelve hours apart, the third value is still blank, until thetwenty-fourth reading is made. At the twenty-fourth hour T₂₄, the nextpacket transmitted includes readings R₂₄, R₁₂, and R₀, as well as anelapsed time indicator. Following the twenty-fifth reading at intervalT₂₅, the transmitted packet contains readings R₂₅, R₁₃, and R₁, and anelapsed time indicator for R₁. This process continues as readings aremade and transmitted. In each case, the packet would include the elapsedtime from the time the MIU recorded that the earliest (oldest) readingwas made and the time of transmission of the packet. Then, once a newerreading was made, the packet includes this newer reading, the readingtaken twelve hours before, and the reading taken twenty four hoursbefore the most recent reading. In this embodiment, if there were anobstruction preventing successful transmission or receipt of a packet,the obstruction would have to last more than twenty-four hours for datato be lost. Further, even though each reading is transmitted only everytwelve hours, it is still transmitted three times before it progressesthrough the available data fields in the packet and rolls off.

As noted, in the embodiment described above, the elapsed time indicatorreferenced the time elapsed since the earliest (oldest) reading in thepacket was made, and the elapsed times for the remaining packets werecomputed based on this elapsed time indicator and the number of readingintervals between the reported readings in the packet. Other embodimentsmay utilize the time elapsed since the most recent reading and thenumber of reading intervals as a basis for computation, or may utilizealternative forms of elapsed time indicators, such as an absolute orrelative time value associated with each reading in the packet.

The interval at which meter readings are made is independent of theinterval at which data packets containing the readings are transmitted.In some embodiments, there may be multiple transmissions between everyreading. For example, the MIU may read the meter once per hour andtransmit a packet containing selected readings every ten minutes. Inother embodiments, there may be multiple readings between transmissions.For example, the MIU may read the meter once per hour and transmit alarge packet of data with many meter readings every twelve hours ortwenty four hours. The principles of the present invention in which thedata packet contains at least two nonsequential readings separated by amultiplicity of reading intervals can be implemented in anyconfiguration of reading and transmission intervals. In a preferredembodiment, the transmission interval is pseudo random to minimizecollisions of data during transmission.

A utility or systems operator is often interested in the current flowrate at a meter. Some meters may actually report a flow rate, not merelya reading of the volume that has passed through the meter. Most meters,however, report only a simple meter reading, and average flow rates mustbe calculated by analyzing volume through the meter over a given timeperiod. In a preferred embodiment, the MIU reports in its data packetthe two most recent meter readings, in addition to at least twononsequential readings, as described above. In this way, this embodimentof the present invention provides information from which the mostcurrent flow can be computed, as well as protecting against data losswhen transmission or receipt of the data packets is temporarilycompromised. Moreover, a data packet containing both the two most recentreadings, as well as nonsequential readings separated by an extendedtime period, allows computation of a current average flow rate and anaverage flow rate over extended period, from the same data packet. Forexample, if meter readings were made every fifteen minutes, and thepacket contained the two most recent readings, as well as readings madetwelve hours and twenty-four hours before the most recent reading, thenthe average flow rates over the fifteen-minute period, the twelve-hourperiod, and the twenty-four hour period preceding the most recentreading could be calculated from the same packet.

In all such readings, however, it is desirable to know that accuratetimes of the meter readings are being reported. As described above, thedata packets include an elapsed time indicator from which the time eachmeter reading in the packet was made can be determined. However, if theclock of the MIU is inaccurate, whether fast or slow, then the MIU'stime stamp for each reading and the elapsed time reported for eachreading also will be inaccurate. One aspect of the present invention isto quantify the extent of any inaccuracy in the MIU clock. As describedin the background section above, the clock in a collector or otherintermediate device receiving transmissions from an MIU is typicallymuch more accurate than the internal clock of the MIU. The clock in thecollector may, for example, be set by using a GPS signal. Adding a GPSreceiver to an MIU would not likely be cost effective but including aGPS receiver in a collector may be worthwhile given the relativelysmaller number of collectors used (in a multi-tiered architecture). Inone embodiment, quantifying the inaccuracy of the MIU clock is done bycomparing the elapsed time reported by the MIU for the same reading oversuccessive transmissions with the actual time elapsed for the reading,based on the clock in the receiving device. The term “same reading” isused to refer to successive packets containing retransmission of thesame actual reading made at a specific time, such as the reading made at12:00 PM on January 1. That same reading will be transmitted multipletimes after it is made; the number of times and interval betweentransmission depends upon the configuration of the system. This aspectof the invention can be applied even when a reading is reportedsequentially, on in successive packets, or nonsequentially when the timebetween reports is rather small.

In one embodiment, the elapsed time for a specific meter reading R₀ isreported by an MIU at time 0, which shall be referred to as reportedelapsed time (RET) RET_(R0-0). When that packet is received, since it isthe first time that reading is received, the collector records theelapsed time reported and actual elapsed time (AET) AET_(R0-0) as thesame value. (In practice, the collector may have already determined theamount by which the reported elapsed time should be adjusted from prioranalysis, as described below). Some time later, the same reading R₀ isagain included in a packet and an updated reported elapsed time isreported (or can be computed) based on the elapsed time indicator in thepacket. In a preferred embodiment, the R₀ reading is included in packetstransmitted twelve hours (with reported elapsed time RET_(R0-12)) andtwenty-four hours (with reported elapsed time RET_(R0-24)) after itsinitial transmission. Upon each successive receipt of the same readingR₀, the actual elapsed time is recorded, which in this example would bedenoted AET_(R0-12) and AET_(R0-24). The reported elapsed time RET iscompared to corresponding actual elapsed time AET to determine how muchthe MIU's clock varies from the correct time.

In a preferred embodiment, this process is repeated each time the samereading is received successively, and is repeated for substantially allreceived readings. The data is recorded and analyzed and the variabilityof the MIU clock is determined. Further, the rate of change of thevariability or drift of the MIU clock may be determined from analysis ofreadings over an extended period of time. For example, when comparingthe elapsed time for same reading as reported over a twenty-four hourperiod, the MIU clock may have a one-percent error. But one month later,the variability over a twenty-four hour period may be 1.5%, and then onemonth later, 2.0%. This data indicates that the accuracy of the MIUclock is decreasing steadily by about 0.5% per month. This could be dueto temperature variations or a component that is not keeping timeproperly. Analysis of the data over a longer time period might allowidentification of the root cause of the problem.

In another embodiment, inaccuracies of the MIU clock that exceed acertain threshold are reported to a central server or system. This maytrigger a visit by a field technician to replace an MIU or lead todiscovery of a manufacturing defect causing such variability. Such athreshold could be based upon a fixed differential between reported andactual elapsed time (such as a one-hour variance over a twenty-four hourperiod) or upon an acceleration in the variance of the MIU clock versusactual time (such as the inaccuracy of the MIU clock progressivelyincreasing by 0.1% per week). Data related to environmental conditions,such as temperature, precipitation, humidity, barometric pressure, andthe like, for the geographic area in which the subject MIU is locatedmay be acquired. By comparing MIU accuracy to these data, the effect, ifany, of various environmental conditions on MIU accuracy may bemeasured.

Some AMR systems utilize two-way communications between the MIU and thecollecting devices. In one embodiment of the present inventionimplemented in such systems, once the error in the MIU clock has beenquantified, a command may be issued to the MIU to reset its clock to thecorrect time to correct the error. If a rate of change of thevariability has been measured, then a calibration factor may be sent tothe MIU, which is used by the internal software of the MIU to compensatefor the error and adjust the MIU clock or reported readings of the clockaccordingly to improve its timekeeping functions.

Although the present invention has been described and shown withreference to certain preferred embodiments thereof, other embodimentsare possible. The foregoing description is therefore considered in allrespects to be illustrative and not restrictive. Therefore, the presentinvention should be defined with reference to the claims and theirequivalents, and the spirit and scope of the claims should not belimited to the description of the preferred embodiments containedherein.

What is claimed is:
 1. A method of determining a variance between ameter interface unit (MIU) clock and an actual time in an automatedmeter reading (AMR) system, comprising: (a) receiving a first packet ofdata from a meter interface unit (MIU) in communication with a utilitymeter by the AMR system, said first packet containing a first reading ofsaid meter, said first reading being associated with a first elapsedtime indicator reporting a first time elapsed since said first readingwas taken based on an MIU clock of said MIU; (b) associating the firstreading in said packet with a first actual time said reading was takenbased upon a receiving device clock of said receiving device and saidfirst elapsed time indicator; (c) receiving a later packet from the MIUby the AMR system, said later packet containing the first reading, thefirst reading being associated with a second elapsed time indicatorreporting a second time elapsed since said first reading was taken basedon the MIU clock, the second elapsed time indicator of said firstreading in said later packet being different than the first elapsed timeindicator; (d) associating the first reading in the later packet with asecond actual time said first reading was taken based upon the receivingdevice clock and said second elapsed time indicator; (e) determining thevariance between the MIU clock and the actual time in the AMR systembased on a difference between the first actual time and the secondactual time; and (f) when the variance exceeds a threshold variancebetween the MIU clock and the AMR system, sending a calibration factorto the MIU to compensate for the variance between the MIU clock and theactual time in the AMR system.
 2. The method of claim 1, comprisingreporting the variance to a central server or system.
 3. The method ofclaim 1, comprising reporting to a central server or system when thevariance exceeds the threshold variance.
 4. The method of claim 3,wherein the threshold variance is based upon a fixed differentialbetween the first actual time and the second actual time.
 5. The methodof claim 3, wherein the threshold variance is based upon an increaseover time of a variance of the first actual time and the second actualtime.
 6. The method of claim 1, comprising issuing a command to the MIUto reset the clock of the MIU to correct the clock of the MIU.
 7. Amethod of determining a variance between a meter interface unit (MIU)clock and an actual time as reported by an automated meter reading (AMR)system, comprising: (a) receiving a first packet of data from an MIUthat makes readings of a meter upon reading intervals by the AMR system,said first packet containing a first reading and a second reading ofsaid meter, each said first and second readings being associated with afirst elapsed time indicator of a time elapsed since said first andsecond reading was taken based on an MIU clock of said MIU, wherein thefirst and second readings in said packet are non-sequential, and whereinan elapsed time between the first and second readings is at least tworeading intervals; (b) associating the first and second readings in saidpacket with an actual time said first and second readings were takenbased upon an AMR system clock of said AMR system and said first elapsedtime indicator; (c) receiving a later packet of data from the MIU by theAMR system, said later packet containing a third reading of said meter,said third reading being associated with a second elapsed time indicatorof a time elapsed since said third reading was taken based on the MIUclock, wherein the third reading is a same reading as the first reading,the second elapsed time indicator being different than the first elapsedtime indicator; (d) associating the third reading in said later packetwith a third actual time said third reading was taken based upon the AMRsystem clock of said AMR system and said second elapsed time indicator;(e) determining a variance between the MIU clock and the AMR systemclock based on a difference between the first actual time and the thirdactual time; and (f) when the variance exceeds a threshold variancebetween the MIU clock and the AMR system clock, sending a calibrationfactor to the MIU to compensate for the variance between the MIU clockand the actual time as reported by the AMR system.
 8. The method ofclaim 7, comprising reporting the variance to a central server orsystem.
 9. The method of claim 7, comprising reporting to a centralserver or system when the variance exceeds the threshold variance. 10.The method of claim 9, wherein the threshold variance is based upon afixed differential between the first actual time and the third actualtime.
 11. The method of claim 9, wherein the threshold variance is basedupon an increase over time of a variance of the first actual time andthe second actual time.
 12. The method of claim 7, comprising issuing acommand to the MIU to reset the clock of the MIU to correct the clock ofthe MIU.